The new generation of mobile communications can enhance the data transmission rate up to 100 Mbit/s or even higher and can support services ranging from voice to multimedia services, which include real-time stream-media service. The data transmission rate can be adjusted dynamically according to the requirements of different services in the new generation of communications. In addition, the other characteristic of the new generation of communications is low cost. Thus, high efficiency frequency band technology is needed to realize high data rate and large capacity within limited frequency resources.
MIMO (multiple input multiple output) technology to develop space resources sufficiently can realize multitransmitting and multireceiving with a plurality of antennas, so that it can increase the channel capacity by multiples without increasing frequency spectrum resources and antenna transmission power. In Orthogonal Frequency Division Multiplexing (OFDM) technology as a kind of multicarrier transmission technology, the carriers are orthogonal with each other and thus the frequency resources can be used efficiently; in addition, OFDM divides the total bandwidth into several narrowband subcarriers, which can counteract frequency selective fading effectively. Therefore, the trend is to combine the two technologies adequately to realize data communications of the next generation mobile communication.
Currently, there are three basic kinds of error control techniques for reliable transmission of data communications, which are Forward Error Correction (FEC), Automatic Repeat reQuest (ARQ) and Hybrid ARQ (HARQ) which is the combination of the FEC and ARQ. HARQ has the best reliability and throughput performance.
HARQ can be classified into three kinds, i.e., HARQ-I, HARQ-II and HARQ-III. HARQ-II and HARQ-III make use of coding and diversity combining respectively and obtain certain coding and diversity gain accordingly.
It should be understood that the traditional HARQ-III combines the different copies received at different time to obtain the time diversity gain. However, the time diversity gain depends on the correlation between the data retransmission intervals and the channel coherence time. When the data retransmission interval is larger than the channel coherence time, the time diversity gain will be significant, otherwise feeble. In order to obtain significant time diversity gain in slow fading channels, therefore, the data retransmission interval must be larger than the channel coherence time, which results in the increase in round-trip delay and badness for real-time services; while in fast fading channels, time diversity gain is more significant.
Hiroyuki Atarashi, et al., “Partial Frequency ARQ System for Multi-Carrier Packet Communication”, IEICE TRANS. COMMUN., VOL. E78-B, No. 8 Aug. 1995 and Liyu Cai, et al., “Improved HARQ scheme using channel quality feedback for OFDM system,” Vehicular Technology Conference, 2004. VTC 2004-Spring. 2004 IEEE 59th Volume 4, 17-19 May 2004 Page(s): 1869-1872 Vol. 4 both disclose a kind of partial retransmission diversity ARQ scheme for OFDM system, wherein a threshold level is set at the receiver side and compared with the level of the signal received on each subcarrier. If the received data is determined error, the data on the unreliable subcarriers whose receiving level is lower than the threshold level will be retransmitted. During retransmission, some better subcarriers can be chosen to retransmit data on those unreliable subcarriers and also all subcarriers can be chosen to re-transmit data on those unreliable subcarriers. Then, the re-transmitted data and the previous received data will be combined at the receiver in order to obtain the time and frequency gain. If only partial subcarriers are used for retransmission, the rest subcarriers can be used to transmit new data.
Hiroyuki Atarashi et al., “An efficient ARQ Scheme for Multi-Carrier Modulation Systems Based on Packet Combining,” IEICE TRANS. COMMUN., VOL. E82-B, NO. 5 MAY 1999 and T. Kumagai, et al., “A maximal Ratio Combining Frequency Diversity ARQ Scheme for High-Speed OFDM Systems,” Personal, Indoor and Mobile Radio Communications, 1998, The Ninth IEEE International Symposium on Volume 2, 8-11 Sep. 1998 Page(s): 528-532 vol. 2 both disclose a frequency diversity ARQ scheme in which the subcarrier allocation mode is changed in retransmission according to the characteristic, the channels on subcarriers whose interval is larger than the coherence bandwidth are independent each other. The retransmission data are allocated to the aforementioned subcarriers in different modes in order to counteract the time correlation on channels in the slow fading environment. And the multiple received copies at the receiver will be combined by maximal ratio combining to obtain the time and frequency diversity gain. The disadvantage of the ARQ scheme is that the subcarriers allocation mode used for retransmission depends on the coherence bandwidth so the allocation manners are limited. In addition, the fading of each subcarrier is similar in the fast fading environment in view of statistics, so the frequency diversity is not as obvious as that in the slow fading environment.
E. N. Onggosanusi, et al., “HARQ Transmission and Combining for MIMO Systems,” Communications, 2003. ICC '03 IEEE International Conference on Volume 5, 11-15 May 2003 Page(s): 3205-3209 vol. discloses a HARQ scheme focusing on MIMO system, which combines the different trellis-coded modulations for retransmission with the antenna permutation, obtains mapping diversity using different trellis-coded modulations and obtains the space gain using the antenna permutation. The scheme is similar with OFDM system, which counteracts the time correlation on slow fading channels by antenna permutation. However, the disadvantage is that the space diversity gain by the antenna permutation is not very significant in the fast fading environment. And if the scheme is used in MIMO-OFDM system, the space-frequency-time diversity gain cannot be developed simultaneously.
Table 1 further shows and compares the characteristics of the four HARQ schemes from the point of subcarrier allocation mode and antenna permutation. Scheme I is a simple combination of chase combining and MIMO-OFDM system. Scheme II and Scheme III implement this two ARQ schemes in single antenna system to each data stream of the MIMO-OFDM system, i.e., data streams of every antenna can adopt different subcarrier modes while the antenna remains unchanged. Compared with Scheme II and Scheme III, the subcarrier allocation modes are fixed for each antenna while the antenna permutation is variable in Scheme IV.
TABLE 1Characteristics of Four HARQ SchemesSubcarrierAntennaAllocationPermu-SchemeNameModetationProblemsSchemeEach antennaFixedFixedNo space andIadopts chasefrequency diversitycombing HARQgainSchemeEach antennaVariableFixedLimited diversityIIadopts partialgain on frequency,frequency ARQno space diversitygain, dependanton SNR thresholdvalue, large feed-back informationSchemeEach antenna4 kindsFixedLimited diversityIIIadopts MRCvariablegain on frequency,frequencymodesno space diversitydiversitygainARQSchemeAntennaFixedVariableLimited diversityIVpermutaiongain on space, noHARQfrequency diversitygain
Aiming at MIMO-OFDM system, it is necessary to design a new kind of HARQ scheme, which can develop frequency-space-time resources adequately.